Final answer:
Betsey Johnson is the American fashion designer known for her whimsical designs and for performing a cartwheel at the end of her shows.
Step-by-step explanation:
The American fashion designer known for her whimsical design aesthetic and for performing a cartwheel after each of her runway shows is Betsey Johnson. Betsey Johnson's style is characterized by its feminine and over-the-top designs that often incorporate bright colors, unique details, and a touch of playfulness. She carved a niche for herself in the fashion industry with her exuberant approach to design, which has become her signature style throughout the years. Her end-of-show cartwheels became an anticipated tradition and a symbol of her youthful and energetic spirit.